1. Field of the Invention
The present invention relates to a method of controlling the writing and erasing of information in flash memories and the like used as an external storage device for personal computers. This type of memory is incapable of having information written over existing information.
2. Description of the Prior Art
In recent years, there has been wide attention on external storage devices which use flash memory. The flash memory does not require a backup power supply because of its non-volatile characteristic, can be rewritten electrically, and, also, is inexpensive. However, the flash memory has shortcomings as follows.
First, it is impossible to write information over existing information. Also, it is impossible to erase information by the unit of byte, but possible by the unit of sector, block, or chip. Thus, a rewrite can not be done byte by byte as in the conventional memory so that the rewriting speed as well as the erasing speed is relatively slow compared to the reading speed.
Second, there is a limit in the number of erasures that the flash memory can tolerate, and a typical flash memory can not be used after a hundred thousand to one million erasures. Thus, areas which experience a larger number of erasures become defective faster than other areas so that the total area available for storing information decreases unless the numbers of erasures are roughly averaged across all the sectors or all the blocks, whichever is used as the unit of erasing.
Since the flash memory has shortcomings as listed above, various counter measures should be taken in using the flash memory, which are ranging from preparing an evacuation area and evacuating data at the time of rewriting, providing a control table for controlling the writing and erasing of information, to preparing a way to save the situation when defective sectors or defective blocks are generated.
As described above, the flash memory has such advantages as a non-volatile characteristic and electrical rewrite capability, but also has many inadequacies as well, so that those inadequacies must be surmounted before using it for a practical purpose.
Accordingly, there is a need in the flash memory field for a method of writing, erasing, and controlling a memory so that those shortcomings are obviated to facilitate use of flash memory for practical purposes.